To provide a method and a device for the production of a long-period grating having accurate characteristics without using a light intensity modulating mask.

When a long-period grating 5 having the modulation distribution of refractive indices is produced by irradiating an optical waveguide 10 with light beams 1, the following processes are carried out so that the refractive index in a specified part of the optical waveguide 10 is induced corresponding to the light which irradiates the optical waveguide. The processes are to alternately generate light passing through for a specified time 2 and light cut for a specified time 3 by an optical switch 15, and to move the irradiation area by the light beam 1 along the axial direction of the optical waveguide 10 by a moving means 14.